The von Hippel Lindau/hypoxia-inducible factor (HIF) pathway regulates the transcription of the HIF-proline hydroxylase genes in response to low oxygen
ABSTRACT Most of the genes induced by hypoxia are regulated by a family of transcription factors termed hypoxia-inducible factors (HIF). Under normoxic conditions, HIFalpha proteins are very unstable due to hydroxylation by a recently described family of proline hydroxylases termed EGL-Nine homologs (EGLN). Upon hydroxylation, HIFalpha is recognized by the product of the tumor suppressor vhl and targeted for proteosomal degradation. Since EGLNs require oxygen to catalyze HIF hydroxylation, this reaction does not efficiently occur under low oxygen tension. Thus, under hypoxia, HIFalpha escapes from degradation and transcribes target genes. The mRNA levels of two of the three EGLNs described to date are induced by hypoxia, suggesting that they might be novel HIF target genes; however, no proof for this hypothesis has been reported. Here we show that the induction of EGLN1 and -3 by hypoxia is found in a wide range of cell types. The basal levels of EGLN3 are always well below those of EGLN1 and EGLN2, and its induction by hypoxia is larger than that found for EGLN1. The inhibitor of transcription, actinomycin D, prevents the increase of EGLN3 mRNA induced by hypoxia, indicating that it is due to enhanced gene expression. Interestingly, EGLN1 and EGLN3 mRNAs were also triggered by EGLN inhibitors, suggesting the involvement of HIFalpha in the control of its transcription. In agreement with this possibility, pVHL-deficient cell lines, which present high HIF activity under normoxia, also showed dramatically increased normoxic levels of EGLN3. Moreover, the overexpression of an oxygen-insensitive mutant form of HIFalpha resulted in increased normoxic levels of EGLN3 mRNA. Finally, hypoxic induction of EGLNs was not observed in cells lacking functional HIFalpha.
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ABSTRACT: The biology of the α subunits of the hypoxia-inducible factors (HIFα) has expanded in the past years from their original role in angiogenesis to their nowadays position in the self-renewal and differentiation of stem cells. Hypoxia is a physiological condition in different tissues-including tumors-and, may cause stem cells in the onset of genomic instability, this last associated in the scientific literature with the acquisition of a malignant phenotypes. HIFα proteins have been the subjects of excellent scientific contributions in the past years, providing new paradigms in the biology of these key proteins and their pivotal role in cell homeostasis. Over other therapeutic implications, the relevance of studies focused on the etiology of tumor-initiating cells and the characterization of the mechanisms that could lead to their malignancy, is gaining significance in the health areas of cancer and regenerative medicine.Vitamins & Hormones 01/2011; 87:367-79. DOI:10.1016/B978-0-12-386015-6.00036-6 · 1.78 Impact Factor
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ABSTRACT: Exposure to limiting oxygen in cells and tissues induce the stabilization and transcriptional activation of the hypoxia-inducible factor 1 alpha (HIF-1α) protein, a key regulator of the hypoxic response. Reactive oxygen species (ROS) generation has been implicated in the stabilization of HIF-1α during this response, but this is still a matter of some debate. In this study we utilize a mitochondria-targeted antioxidant, mitoubiquinone (MitoQ), and examine its effects on the hypoxic stabilization of HIF-1α. Our results show that under conditions of reduced oxygen (3% O2), MitoQ ablated the hypoxic induction of ROS generation and destabilized HIF-1α protein. This in turn led to an abrogation of HIF-1 transcriptional activity. Normoxic stabilization of HIF-1α, on the other hand, was unchanged in the presence of MitoQ suggesting that ROS were not involved. This study strongly suggests that mitochondrial ROS contribute to the hypoxic stabilization of HIF-1α.FEBS Letters 05/2005; 579(12):2669-2674. DOI:10.1016/j.febslet.2005.03.088 · 3.34 Impact Factor
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ABSTRACT: Structural and functional integrity of brain function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of brain function. In particular, reductions in oxygen availability (hypoxia) caused by systemic or local blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply to the brain by anaerobic glycolysis. Hypoxia has been implicated in central nervous system pathology in a number of disorders including stroke, head trauma, neoplasia and neurodegenerative disease. Complex cellular oxygen sensing systems have evolved for tight regulation of oxygen homeostasis in the brain. In response to variations in oxygen partial pressure (P(O(2))) these induce adaptive mechanisms to avoid or at least minimize brain damage. A significant advance in our understanding of the hypoxia response stems from the discovery of the hypoxia inducible factors (HIF), which act as key regulators of hypoxia-induced gene expression. Depending on the duration and severity of the oxygen deprivation, cellular oxygen-sensor responses activate a variety of short- and long-term energy saving and cellular protection mechanisms. Hypoxic adaptation encompasses an immediate depolarization block by changing potassium, sodium and chloride ion fluxes across the cellular membrane, a general inhibition of protein synthesis, and HIF-mediated upregulation of gene expression of enzymes or growth factors inducing angiogenesis, anaerobic glycolysis, cell survival or neural stem cell growth. However, sustained and prolonged activation of the HIF pathway may lead to a transition from neuroprotective to cell death responses. This is reflected by the dual features of the HIF system that include both anti- and proapoptotic components. These various responses might be based on a range of oxygen-sensing signal cascades, including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low P(O(2)) affinity cytochrome c oxidase (aa(3)) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF prolyl-hydroxylase (PHD) and HIF asparaginyl hydroxylase, known as factor-inhibiting HIF (FIH-1). Thus specific oxygen-sensing cascades, by means of their different oxygen sensitivities, cell-specific and subcellular localization, may help to tailor various adaptive responses according to differences in tissue oxygen availability.Journal of Experimental Biology 09/2004; 207(Pt 18):3171-88. DOI:10.1242/jeb.01075 · 3.00 Impact Factor